When you drained the wort from the grains, what was the volume of the run-off? This measurement will greatly affect the results of the Analysis. Also, did you mash the grains in 4 gallons of water? 4 pounds of grain should only require about 1 gallon of water for a single step infusion. Also, is the cooler fitted with a false bottom or drainage manifold? This device collects run-off evenly from the entire filter bed.
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I noticed in the original post he stated that he drained the wort with the valve wide open. It's been my experience that you must drain the wort very slowly to prevent channeling through the grainbed, perhaps this could explain some of the problem.....In MHO:)